The cunning ones kneel before six masters, but serve only one.
It is clear the dreadlords are all playing the various cosmic forces. Now the only question is who do they actually serve? Initially one would think the Legion but more likely they serve the void.
Denathrius and the Jailer.
The text makes it pretty clear they were created by, and serve, Death.
The text literally says they infiltrate the demons and spread lies among the Void lol
They serve Death™
But I’m betting on a Dark First One™
But who is to say they are not lying? Playing double agent to the Jailer sounds like something they would do. Especially if we end up beating the Jailer this expansion.
And then when the Jailer inevitably goes down, it’ll be revealed that they were actually working for someone else, and the the cycle will continue.
Sylvanas because she existed before time.
I believe if one reads those words a certain way… it may mean that they serve “one” who is altogether different than the 6.
I guess a silly example would be if some lothario said the phrase: “I am engaged to 6 women but I am only married to one.”
In such a phrase, the wife would be a 7th woman, different from the 6 women the hypothetical lothario is engaged with.
Sa-ru-maaannnnnn
Serving the Jailor would still be serving Death.
I can think of no more stealthy operator than a giant horned demon with bat wings and claws.
Oh, see that demon over there? He’s our new friend.
It brings to mind that clip of Steve Buscemi with the skateboard and backwards baseball cap saying “Hello, fellow kids!”
It’s a motif that fits in pretty well with 3 of the factions. And 2 of the remaining 3 are the stupid ones.
Jesus. 
/10chars
That’s why I think it’s a Dark First One but its not so much double agent as recognizing the root of their creator’s power.
You mean WoW-Morgoth 
Medivh + Denathrius is WoW Saruman (former of which might be good and alive again now), Zovaal is WoW Sauron, Void Lord is WoW Ungoliant, Sargeras is WoW Gothmog, and we are lacking the Dark First One WoW Morgoth.
Sire Denathrius. It’s basically implied that he made them.
The thing is the book was found at Sinfall, which was Denathrius’ personal workshop that none of the other Venthyr were allowed to enter. And from the cinematic we know that the way the book is written is how he talks.
Expansion 9!
in D&D there ar esome gods that are extremely primordial.
Gods that just happen to be out of a concept or idea.
Like the concept of death becoming a scentient entity or life itself.
Hard to explain but cosmic fundamental ideas just came into existance.
“First ones” might be something like that.
Concepts that are as old as creation itself which are part of creation as an idea.
Like Death, Life, Light and Dark. Love and Hate maybe. Destruction and Creation.
So if that is the case, dreadlords and even the Jailer might be linked to the concept of death itself.
What evidence do they have that they are lying? I mean, they we’re sent out to do exactly what they have done, which is to infiltrate the various factions and report back to their original master (IE, presumably the Jailor or Denathrius.), which, by the leak we recieved, they have been doing. Yes, it’s possible they were giving false information, but just from a story perspective, that makes the twist of revealing their origins pretty redundant. The idea that they serve a master in the Shadowlands and aren’t actually native to the living universe was the big reveal, IMO.
That being said, if we kill off the Jailer, it seems probable they would find someone else to serve, possibly the void, given their various connections. Or one of them might step forward to lead their race and continue their quest to manipulate all ends.
Something interesting happened. So the Fel right we don’t know who their dark entity is and no it isn’t Sargeras. it wasn’t mentioned but it is hinted that someone rules the twisting nether i believe.